The present invention relates to capacitive acoustic microphones and, more particularly, to capacitive acoustic microphones involving a first field plate mounted to a semiconductor substrate structure and a second metal field plate mounted to the semiconductor substrate structure so that the first and second field plates form an acoustically responsive capacitor. The invention also involves the method of fabricating such a capacitor.
Various capacitive microphones have been known in the prior art. Typically, these microphones are either constructed from discrete, non-semiconductor materials which do not lend themselves to semiconductor batch fabrication or constructed by use of semiconductor batch fabrication technology but without the advantages that nontraditional semiconductor fabrication can provide.
In the former case, the diaphragm electrode of the capacitive transducer is mechanically stretched over a void created in a base plate which is typically an insulative base plate such as alumina. A second electrode is then plated to the insulative base plate to form the second plate of the capacitor. The process of making such a transducer is time consuming and cannot take advantage of semiconductor processing which can, as an example, fabricate both the transducer and the electronic signal processing circuitry on the same semiconductor substrate.
In the latter case, where semiconductor batch fabrication techniques are used for making the transducer, both the diaphragm electrode and the stationary fixed electrode are constructed from semiconductor materials. Such arrangements are expensive to construct and do not yield optimum capacitive type transducers.